"One of our largest space bombers was over the planet. The commander warned the Aldebarans that unless they agreed to surrender the diamonds at once, he would drop a planet
buster - that’s a Class-A oxygen bomb.
The Aldebaran’s return message, expressed in the crudest possible language,
told the commander what he could do
with the bomb.”

The reference to a "oxygen bomb" is a satiric reference to the hydrogen bomb.

I first encountered this term in Frank Herbert's quirky but excellent 1972 novel The Godmakers:

We'll take up a tight orbit. Out beyond us will be five transports full of I-A marines plus a Class IX Monitor with one planet-buster. You're calling the shots, God help you! First, we have to know if they've taken the Delphinus , and if so, where it is. Next, we want to know how warlike these goons are. Can we deal with them? Are they too bloodthirsty? What's their potential?